This invention relates generally to computer systems and in particular to peripheral devices which are connected to a computer system via a bus.
As it is known in the art, computer systems generally include at least one central processing unit (CPU), a memory, and some type of mass storage device such as a disk drive. A typical computer operates by reading and executing program instructions which have been read from a mass storage device and placed in the memory.
As is also known in the art, mass storage devices are typically coupled to a computer system via bus which operates in accordance with a so called bus protocol. The protocol defines the nature of the signals used to produce a proper interface between the computer system bus and the mass storage device. One such protocol in wide use today is the so called Small Computer System Interface (SCSI).
The Small Computer System Interface is a widely implemented standard for transferring data between a host computer and a peripheral device such as magnetic hard disk, streaming tape backup mechanism, CD-ROM drive, or other peripheral device. The SCSI standard allows up to seven devices to be connected to the host in a so called daisy-chain configuration. Each device in the chain has a unique ID, numbered 0 through 6 (ID 7 is reserved for the host).
Many business applications, such as multimedia development or database management, run on typical computer systems require access to extremely large data Fries typically stored on commensurately large capacity (e.g. greater than one gigabyte) storage devices. Although these large capacity storage devices provide adequate storage solutions, their cost can be prohibitively expensive. As such, many small businesses will own as few as one such device and share it among many users.
The modern trend in accommodating many users sharing a single large capacity storage device is through the use of a network. With a network, several computer systems are linked either together or to common resources such as high capacity storage devices or both. Although this arrangement provides a measure of convenience for each user, it can also be prohibitively expensive since each computer connected to a network requires additional hardware and software to facilitate connection to the network. Additionally, at present network technology, data transfer rate over a network does not match the rate at which data can be transferred to and from a processor via a storage device directly connected to the computer via a bus such as the SCSI bus described above. As an example, graphics departments, multimedia developers, and database creators routinely share large files (50+MB). The network approach to snaring the files typically requires the use of a local area network (LAN) such as Localtalk, Ethernet, or Novell, etc. These networks transfer data at a relatively slow (10 Mbit/sec) transfer rates. The slow transfer rate of the currently implemented network protocols degrades performance on both sending and receiving computers (and fileservers, if used) while the file transfer takes place. With these networks, the transfer of a typical file from the large capacity device can range from 10 to 60 minutes or more, depending on file size. In addition, other network traffic suffers while these large files are sent
Another approach involves the use of a removable media storage solution. The removable media may be for example, a hard disk, optical or magneto-optical mechanism that is designed to accept storage media in the form of a removable cartridge. This facilitates the exchange of large amounts of data between users who have such a mechanism connected to their computer. In general, the transfer rate of removable media drives is comparable to that of the SCSI interface. However, the cost of placing such a mechanism on every users desktop can be prohibitive to many smaller business As a result of the above described limitations, many businesses have resorted to swapping large capacity storage devices among multiple computer systems. That is, when one user requires access to one of the large files stored on a large capacity storage device, the device will be physically disconnected from one computer and reconnected to the system needing access to the data files. Although physically moving devices among several computer systems alleviates many of the problems described above, this technique is not without its own drawbacks.
As stated in most user manuals supplied with computer systems or adapter cards incorporating the SCSI bus protocol and manuals supplied with peripheral devices designed to operate in accordance with the SCSI protocol, there are certain requirements which must be followed when inserting or removing SCSI devices from the SCSI chain.
Before physically connecting (or removing) a device to the SCSI chain (i.e., plugging the SCSI cable in), all devices in the chain, including the host, must be turned off. This requirement is due to the design of the SCSI controllers used in both the host computer and the target SCSI devices. Failure to adhere to the stated requirements may result in permanent electrical damage to both the host system and the peripheral device.
This limitation of powering down all devices (including the computer) before adding or removing a device from the SCSI chain prevents the easy sharing of a SCSI device between different computers. Users must stop any work in progress, power down the system, attach a SCSI device (obeying rules of SCSI termination) and then restart the computer. To share a single hard disk, this requires two shutdowns and two power-ups as the peripheral device is removed from the first computer (after being powered down first) attached to the second computer (after being powered down first) and then both computers restarted. The result is a complete interruption of all work in profess on both computers, and the time required for the shut-down/restart cycles. In practice, this means devices that could be shared (spare hard dish, tape backups, etc.) are not shared due to the inconvenience.
Additionally, in order to conform to the SCSI bus protocol, each end of the daisy chain arrangement needs to be physically terminated. Termination of a SCSI device typically involves providing a 330 ohm resistance between each signal line and a ground potential, and a 220 ohm resistance between each signal line and +5 v. For SCSI devices designed to be mounted internally within the CPU enclosure termination is typically supplied via a number of single in-line resistor packages (SIPs) which are socketed to a circuit board associated with the internal device. External SCSI mechanisms (i.e. those having their own power supplies, enclosure, etc.) may be configured to provide termination in a similar manner. However, to facilitate the daisy chaining of external peripheral devices the termination is often provided in the form of a connector which is coupled to an external port of the last SCSI device in the SCSI chain. The connector includes similar SIPs as those used with internal devices and thus provides the required termination. This type of so called "passive termination" does not always provide the proper termination parameters for the SCSI bus, particularly in complex, multiple-device chains.
To provide improved termination, a digital active termination circuit may be employed. Digital active termination provides a way to actively regulate the voltage and impedance on all the SCSI data and control lines. However, digital active termination alone does not provide any protection to the devices on the SCSI chain during addition or removal of a device from the active (powered) chain.